How the heck does serum activate Akt?!?

I've been searching and searching for literature on reasons why I didn't see Akt activation in response to VEGF stimulation in HUVECs. The only thing I can come up with is that I didn't serum starve the cells prior to stimulation. Growth factors in the serum activate pathways that are Akt dependent? That's my guess but I can't find anything to back it up. Any insight?
Thanks,
Katie